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Effect of hot-humid exposure on static strength of adhesive-bonded

aluminum alloys

Rui ZHENG

a

, Jian-ping LIN

a,

*

, Pei-Chung WANG

b

, Yong-Rong WU

a

a

School of Mechanical Engineering, Tongji University, Shanghai 201804, China

b

Global Research& Development Center, General Motors Corporation, Warren, MI 48090-9055, USA Received 30 November 2014; revised 31 December 2014; accepted 19 January 2015

Available online 10 April 2015

Abstract

The effect of hot-humid exposure (i.e., 40 C and 98% R.H.) on the quasi-static strength of the adhesive-bonded aluminum alloys was studied. Test results show that the hot-humid exposure leads to the significant decrease in the joint strength and the change of the failure mode from a mixed cohesive and adhesive failure with cohesive failure being dominant to adhesive failure being dominant. Careful analyses of the results reveal that the physical bond is likely responsible for the bond adhesion betweenL adhesive and aluminum substrates. The reduction in joint strength and the change of the failure mode resulted from the degradation in bond adhesion, which was primarily attributed to the corrosion of aluminum substrate. In addition, the elevated temperature exposure significantly accelerated the corrosion reaction of aluminum, which accelerated the degradation in joint strength.

Copyright© 2015, China Ordnance Society. Production and hosting by Elsevier B.V. All rights reserved.

Keywords: Epoxide adhesive; Aluminum alloy; Hot-humid exposure; Static strength; Bond adhesion

1. Introduction

The use of adhesive is posed to increase dramatically for application to the next generation of vehicle structures as the use of lightweight materials (e.g., aluminum and magnesium alloys) [1e3]. In spite of this, the use of adhesive-bonded aluminum joints in vehicle structures has been limited, mainly due to the degradation of crashworthiness and struc-tural durability caused by the hot-humid exposure[4,5].

In vehicle structures, the majority of the adhesive-bonded components is exposed to the environment of temperature and moist air. Previous studies revealed that if the exposure was over a significant period of time, the joint strength

grad-ually declined [6] which was closely related to the water

absorption of the adhesive-bonded joints[7,8]. Many studies

[9e12]on effect of hot-humid exposure on the strength of the adhesive-bonded aluminum alloys showed that the hot-humid exposure significantly decreased the strengths of the adhesive-bonded aluminum joints. The strength degradation primarily resulted from that water absorption in the interface between adhesive and adherend, which resulted in the surface electro-chemical corrosion of aluminum adherend, and consequently led to the degradation in the interfacial bond between adhesive and adherend. To improve the corrosion resistance of the adhesive-bonded aluminum joints, various surface treatments of aluminum were utilized. Lunder et al.[10]investigated the effect of surface pretreatment of aluminum alloys on the joint strength and found that the improvement in corrosion resis-tance of treated aluminum alloys significantly promoted the durability of the bonded joints. However, although these sur-face pretreatments improved the corrosion resistance of aluminum, there was still a slight decrease in joint strength, * Corresponding author. Tel.: þ86 139 0171 9457; fax: þ86 021 6958 9485.

E-mail address:jplin58@tongji.edu.cn(J.P. LIN).

Peer review under responsibility of China Ordnance Society.

ScienceDirect

Defence Technology 11 (2015) 220e228

www.elsevier.com/locate/dt

http://dx.doi.org/10.1016/j.dt.2015.01.005

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and the treated aluminum in the overlap region was not corroded by the electrochemical reaction. Furthermore, the degradation mechanism of the adhesive-bonded aluminum alloys exposed to hot-humid environment has been still unclear.

In the present study, the effect of hot-humid exposure on the strength of the adhesive-bonded aluminum joints is investigated. The strength of the adhesive-bonded aluminum joint is evaluated using lap-shear joint configuration. Differ-ential scanning calorimetry (DSC), contact angle measure-ment, surface free energy dispersive X-ray spectroscopy (EDS) and polarization corrosion test are utilized to analyze the degradation mechanism of adhesive-bonded aluminum joints exposed to hot-humid environment. Finally, the effect of elevated temperature in hot-humid environment on the corro-sion resistance of the adhesive-bonded aluminum joints is discussed.

2. Experimental 2.1. Materials

1.0 mm thick bare Novelis X610-T4PD (hereafter referred to as X1.0) and 0.9 mm thick bare Novelis X626-T4P (here-after referred to as X0.9) aluminum alloys (here(here-after referred to as XX substrates) were used in this study.L adhesive (i.e., a bi-component epoxy-modified acrylic adhesive containing 0.25 mm diameter glass beads to control the bondline thick-ness) was selected to bond XX aluminum substrates. The main chemical composition of aluminum substrates were measured by X-ray fluorescence spectroscopy (Bruker AXS SRS 3400, Germany), referring toTable 1. The mechanical properties of aluminum substrates provided by supplier are listed inTable 2.

Table 3lists the mechanical properties ofL adhesive provided by supplier.

2.2. Sample fabrication

All substrates were sheared into 25 mm 100 mm samples which were cleaned by 1-1-1 trichloroethane.L adhesive was

applied to the substrates, and the top sheet was set down upon the bottom sheet per the lap-shear joint configuration

(here-after referred to as XXL joint), as shown in Fig. 1. The

adhesive-bonded samples were prepared as follows: (a) applying the adhesive on one of the two adherends using a hand-held injection gun, and positioning the adherends with and without dispensed adhesive using a fixture; (b) bringing the adherends together by a fixture under ambient laboratory conditions, and a pressure was applied via the fixture so that a bondline thickness of 0.25 mm can be maintained; (c) curing the samples in the oven as per the supplier's recommended curing procedure (i.e., 16 h at room temperature and then

20 min at 170 C). The schematic diagram of adhesive

bonding process for aluminum alloys is presented in Fig. 2. All finished samples were examined and the spew fillets around the edge of the overlap were retained to simulate real production conditions.

Table 1

Chemical composition (Wt. %) of Novelis X610-T4PD (X1.0) and X626-T4P (X0.9) aluminum alloys.

Substrate Mg/% Si/% S/% Ti/% Mn/% Al/%

X1.0 0.63 0.82 0.01 0.05 0.12 Balance

X0.9 0.46 1.14 0.01 0.02 0.12 Balance

Table 2

Mechanical properties of Novelis X610-T4PD (X1.0) and X626-T4P (X0.9) aluminum alloys. Substrate Yield strength/MPa Ultimate tensile strength/MPa Total elongation/% X1.0 119.9 228.6 21.9 X0.9 108.7 214.8 22.4 Table 3

Mechanical properties of L adhesive.

Material Yield strength/MPa Tensile strength/MPa Elongation/%

L adhesive 3.7 8.9 28.6

Fig. 1. Configuration of adhesive-bonded lap-shear joint (dimensions in mm).

Fig. 2. Schematic diagram of adhesive bonding process for aluminum alloys (a) Applying adhesive and positioning the aluminum substrates (b) Pressure is applied by the fixture to maintain a bondline thickness of 0.25 mm (c) Curing the samples for 16 h at room temperature and then 20 min at 170C.

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2.3. Differential scanning calorimetry

Differential scanning calorimetry (DSC) has been exten-sively used to investigate the chemical reaction process

[13,14]. In this study, DSC measurement was performed forL adhesive to know the change in the exothermic enthalpy of

adhesive using TA Instruments DSC Q100[15]. For the DSC

measurement of adhesive, the adhesive with a weight of 20e25 mg was placed in a hermetic crucible, and an equiv-alent pan was used as a reference. Also, the adhesive with

similar weight (about 20e25 mg) was placed in a hermetic

crucible with aluminum substrate (i.e., bare Novelis X610-T4PD (X1.0) or X626-T4P (X0.9), aluminum substrate was cut into round to be firstly filled into a crucible), and an equivalent pan with aluminum substrate was uses as a refer-ence. Quantitative dynamic scans ramped at 15C/min from

0 C to 200 C with a heating rate being approximately

equivalent to the rate for the lap-shear joints in curing process.

2.4. Contact angle measurement and surface free energy Good wettability of a surface is a prerequisite for good adhesion [16]. Contact angle is closely related to wettability. For the purpose of investigating the effect on the strength and failure mode of the adhesive-bonded joint, the surface free energy of the substrates was estimated by measuring the contact angle of test liquids on the substrates. Distilled water, diiodomethane and ethylene glycol were used as a probe to measure the contact angle of the aluminum substrates by the sessile drop method with a Dataphysics OCA-20 contact angle analyzer[17]at 23C. The volume of a test drop is 2mL. The liquids were chosen to cover the broadest possible range from highly polar (water) to almost completely dispersion (diiodo-methane). The data of the surface tension and surface tension components of test liquids at 23C are given inTable 4 [18].

This measurement method is based on Young's equation

[19], which describes the condition for equilibrium at a sol-ideliquid interface.

gLcosq¼ gS gSL ð1Þ

where gL is the experimentally determined surface tension of

liquid,q is the contact angle, gSis the surface free energy of

the solid, and gSL is the solideliquid interfacial energy.

In order to obtain the solid surface free energy, gS, an

estimate of gSL, must be obtained. Owens et al. [20]

extended Fowkes's ideas [21] and proposed a geometric

mean approach to combine the dispersion and non-dispersion (polar) interactions, and the following expression for gSL was applied gSL¼ gSþ gL 2 ffiffiffiffiffiffiffiffiffiffi gd SgdL q  2 ffiffiffiffiffiffiffiffiffiffigp Sg p L q ð2Þ Combining Eq.(2) with Eq.(1)yields

gLð1 þ cosqÞ ¼ 2 ffiffiffiffiffiffiffiffiffiffigd SgdL q þ 2 ffiffiffiffiffiffiffiffiffiffigp Sg p L q ð3Þ The contact angle of at least two liquids with known sur-face tension components (gL;gdL;g

p

L) on the solid must be

determined to obtain gdSand gpSof a measured material. In this case, three liquids (i.e., distilled water, diiodomethane and ethylene glycol) were used to determine gdS and gpS of these aluminum substrates.

Bond adhesion between adhesive and substrate is a key for sound joint strength [16], which is related to the work of adhesion. In order to assess the bond adhesion between ad-hesive and substrate, the work of adhesion was calculated with Dupre equation[22].

WAd¼ gSþ gL gSL ð4Þ

where the energy of bond adhesion (WAd) is directly related to

the surface free energy of the two adjacent phases S (sub-strates) andL (adhesive). Combining Eqs.(2) and (4), we have WAd¼ 2 ffiffiffiffiffiffiffiffiffiffi gd SgdL q þ 2 ffiffiffiffiffiffiffiffiffiffigp Sg p L q ð5Þ 2.5. Hot-humid exposure

To simulate the extended exposure in corrosion environ-ment, the bonded joints were exposed to 98% relatively hu-midity and 40C for 240 h. The joints were removed from the

hot-humid chamber shown in Fig. 3 and immediately

quasi-static tested.

Table 4

Test liquids and their surface tension components[18].

Liquids Temperature/C Surface tension data/(mN$m1)

gL gdL g p L Distilled water 23 72.8 21.8 51.0 Diiodomethane 23 50.8 50.8 0 Ethylene glycol 23 48 29 19.0

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2.6. Quasi-static testing

The cured samples were kept at room temperature for 24 h, and then the quasi-static test was performed by loading each sample to failure in Zwick Z050 tensile tester according to the standard ASTM D1002-2001[23]for the determination of the joint strength. To minimize the bending stresses inherent in the testing of single-lap shear samples, filler plates i.e. shims, were attached to both ends of the sample using masking tape to accommodate the sample offset. Load vs. displacement curves were obtained as the samples were loaded at a stroke rate of 10 mm/min. The joint strength is evaluated by using the peak load. Five replicates were performed, and the average peak loads were reported.

2.7. Fractography

Post-failure analysis was performed with an optical mi-croscope to study the failure mechanisms for both as-fabricated and neutral salt spray exposure samples. Fractog-raphy samples were cut from the surface of fractured overlap sections.

2.8. Corrosion resistance

The polarization curves [24] of aluminum substrates in 3.5% NaCl solution at 23 C and 40 C, respectively, were measured to evaluate the corrosion resistance of the X610-T4PD and X626-T4P aluminum. A saturated calomel elec-trode was used as a reference elecelec-trode, and a large-area Pt electrode was used as the assistant electrode. A sample area of 3.125 cm2was exposed to the solution and the scanning speed was 0.8 mV/s.

3. Results and discussion

3.1. Effect of hot-humid exposure on joint strength

Lap-shear joints were fabricated and exposed under hot-humid conditions (i.e., 98% R.H. and 40 C) for 240 h, and then removed for quasi-static testing.Fig. 4presents the effect of hot-humid exposure on the strength of adhesive-bonded lap X610-T4PD and X626-T4P aluminum joints. As shown, the hot-humid exposure decreased significantly the strength of the adhesive-bonded joints. To understand the decrease in joint strength, the fractography of the tested adhesive-bonded joints was observed in Fig. 5. It can be seen that the hot-humid exposure changed the failure mode from a mixed cohesive and adhesive failure with cohesive failure being dominant to adhesive failure being dominant.

The significant degradation in joint strength of the exposed adhesive-bonded aluminum may be attributed to the decline of

the inherent mechanical properties of L adhesive and the

reduction in bond adhesion between the adhesive and substrate or a combination of the two. To understand the cause of the strength reduction, the effect of hot-humid exposure on the

properties of L adhesive and the bond adhesion between

adhesive and aluminum was investigated and will be described next.

3.2. Effect of hot-humid exposure on properties of L adhesive

In order to understand the effect of hot-humid exposure on

the mechanical properties of L adhesive, L adhesive was

carefully removed from the exposed samples and immediately analyzed by differential scanning calorimetry (DSC). Fig. 6

shows the effect of a hot humid exposure on the

thermo-grams of unexposed and exposed L adhesive samples. As

shown, the hot-humid exposure had little influence on the curing extent and glass transition temperature (~40 C) of L adhesive, which means that the adhesive properties were degraded little by the environmental exposure. Therefore, in terms of degrading the joint strength, the adhesive degradation resulting from hot humid exposure was ruled out. These results Fig. 4. Effect of hot-humid exposure on strength of adhesive-bonded 1.0 mm thick X610T4PD and 0.9 mm thick X626-T4P aluminum joints.

Fig. 5. Effect of hot-humid exposure on fractography of adhesive-bonded 1.0 mm thick X610-T4PD and 0.9 mm thick X626-T4P aluminum joints (a) As-fabricated (b) Exposed to hot-humid conditions (i.e., 98% R.H. and 40C).

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indicate that the reduction in joint strength is likely caused by the degradation in bond adhesion between adhesive and aluminum substrates. Liu et al.[25]also found similar results by studying the effect of hot-humid exposure on the durability of adhesive-bonded magnesium AZ31.

3.3. Adhesion characteristic between adhesive and aluminum

Adhesion is the interatomic and intermolecular interaction at the interface of two surfaces[26]. It is well known that the physical or chemical bonds are primarily responsible for the adhesion between adhesive and adherend[16,27]. In order to identify if the existence of chemical bonds between the ad-hesive and aluminum adherends of the adad-hesive-bonded XXL joint, DSC tests of the adhesive and adhesive with aluminum samples were conducted. As a result of the process of forming new chemical bonds accompanied by the heat release, it may induce the exothermic difference between the adhesive and adhesive on the aluminum adherend samples in the curing process. UncuredL adhesive samples positioned in a crucible with a bare aluminum substrate and on bare aluminum cou-pons were tested and the representative results are presented in

Fig. 7. As shown inFig. 7, the DSC results forL

adhesive-in-crucible,L on-Novelis X610-T4PD and L

adhesive-on-Novelis X626-T4P samples in a crucible exhibited little

difference except for the difference in heat flow. The careful examination of DSC analysis results revealed that the differ-ence in exothermic enthalpy (i.e., integral of heat flow and peak area shown in curves 1, 2 and 3 in Fig. 7) was likely caused by the difference in sample weight. These results suggested that little chemical reaction was occurred in the

heating process betweenL adhesive and bare Novelis

X610-T4PD and Novelis X626-T4P aluminum. In other words, the physical bond was likely responsible for the bond adhesion betweenL adhesive and aluminum substrates.

To assess the bond adhesion between adhesive and aluminum adherend, the work of adhesion between adhesive and aluminum adherend was estimated. As indicated in Eq.

(5), to estimate the work of adhesion between adhesive and aluminum adherend, the surface free energies and their com-ponents of adhesive and aluminum were firstly estimated by the measured contact angles of three testing liquids (i.e., distilled water, ethylene glycol, and diiodomethane) upon the adhesive and aluminum adherends.

Table 5lists the contact angles of these test liquids upon the

aluminum adherends. In addition, the contact angles of L

adhesive were measured by dispensing them on a substrate. The measurements were immediately performed as soon as the adhesive was dispensed on the adherends. The results are listed inTable 6.

The surface free energies of XX aluminum adherends were estimated based on the method in Ref.[20]and the results are graphically presented in Fig. 8. As shown, the surface free Fig. 6. Effect of hot-humid exposure (i.e., 40C and 98% R.H. for 10 days) on

the thermograms ofL adhesive from adhesive-bonded 1.0 mm thick X610-T4PD and 0.9 mm thick X626-T4P joints.

Fig. 7. Thermograms ofL adhesive with and without 1.0 mm thick X610-T4PD and 0.9 mm thick X626-T4P aluminum at a rate of 15C/min.

Table 5

Contact angles of X610-T4PD and X626-T4P aluminum adherends. Adherends Distilled water Ethylene glycol Diiodomethane

Contact angle /() Standard deviation /() Contact angle /() Standard deviation /() Contact angle /() Standard deviation /() X610-T4PD 94.3 1.93 79.3 0.74 56.7 1.05 X626-T4P 93.4 1.01 70.9 0.35 54.8 1.20 Table 6

Contact angles ofL adhesive.

Adhesive Distilled water Ethylene glycol Diiodomethane Contact angle /() Standard deviation /() Contact angle /() Standard deviation /() Contact angle /() Standard deviation /() L 55.9 3.20 45.8 0.50 56.6 2.05

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energy of X610-T4PD aluminum was comparable to that of

X626-T4P aluminum. The surface free energy of L adhesive

was estimated, and its dispersion and polar components were 22.25 and 22.02 mJ/m2, respectively.

Based on Eq.(5), the work of adhesion between X610-T4P

and X626-T4P aluminum adherends and L adhesive was

estimated and the results are presented inFig. 9. As shown, the work of adhesion between X626-T4P aluminum adherend and L adhesive was comparable to that between X610-T4P

aluminum adherend and L adhesive. Therefore, it can be

deduced that the bond strengths between the adhesive and X610-T4PD aluminum were similar to that of X626-T4P aluminum.

3.4. Degradation mechanisms

The decrease in joint strength was likely attributed to the degradation of intermolecular physical bond between adhesive

and aluminum. To understand the degradation in strength of the exposed adhesive-bonded joints, the effect of hot-humid exposure on the physical adhesion between adhesive and aluminum is discussed next.

The work of adhesion of the bonded aluminum joint consists of intermolecular van del Waals force (i.e., all intermolecular dispersion force, polar or polar-nonpolar intermolecular orientation force and polar-nonpolar

inter-molecular induction force) and hydrogen bonds [28] which

relates directly to the polar component Wp and dispersion

component Wdof the work of adhesion. When the

adhesive-bonded aluminum joint was exposed to hot-humid condi-tions, the strong polar water molecules primarily destroyed the hydrogen bond and dissolved many polar molecules

[12,29] to decrease the polar component, Wp, and slightly

degraded the dispersion component, Wd, of the work of

adhesion in the interface of the adhesive-bond joint, which would weaken the joint strength without electrochemical reaction. Furthermore, water molecular may diffuse into the

interface between adhesive and substrate [30], which may

lead to the electrochemical reaction of aluminum substrate in the overlap of the adhesive-bonded joint. The electro-chemical reaction equations governing the corrosion of the aluminum substrate are:

Al/Al3þþ3e ð6Þ

O2þ2H2Oþ 4e/4OH ð7Þ

2Hþþ2e/H2[ ð8Þ

To study if the existence of electrochemical reaction for the aluminum exposed to the hot-humid condition (i.e., 98% R.H.

and 40 C), the oxygen contents of the as-received

X610-T4PD aluminum substrate and aluminum substrate on the fracture surfaces from the hot-humid exposed adhesive-bonded joint were examined by energy dispersive spectrom-eter (EDS). The results are shown in Fig. 10. As shown, the

hot-humid exposure increased the oxygen content of

aluminum, which suggests that the aluminum substrate on the fracture surfaces from the exposed joints was corroded under the hot-humid conditions. Similar results were also observed by Vera et al.[31]. The electrochemical reaction changed the surface chemistry and structure of aluminum substrate in the overlap region, and consequently resulted in a degradation in intermolecular force [32] at the interface between adhesive and aluminum substrate, which reduced the dispersion component, Wd, and polar component, Wp, of the work of

adhesion. In other words, the degradation in bond adhesion between adhesive and aluminum was ascribed to the corrosion of aluminum and the polar component of work of adhesion between adhesive and aluminum being destroyed by moisture, which the corrosion of aluminum was dominant.

In addition, temperature had a significant influence on the chemical reaction of aluminum [33]. To understand the effect of temperature in the hot-humid condition on the strength of the adhesive-bonded joint, the corrosion tests of exposed adhesive-bonded aluminum joints were performed.

Fig. 8. Calculated surface free energy of as-received 1.0 mm thick X610-T4PD and 0.9 mm thick X626-T4P aluminum adherends.

Fig. 9. Calculated work of adhesion between L adhesive and aluminum adherends in adhesive-bonded 1.0 mm thick X610-T4PD and 0.9 mm thick X626-T4P joints.

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3.5. Effect of temperature on corrosion resistance of aluminum substrates

To understand the effect of temperature on the corrosion resistance of aluminum substrates, the polarization curves of X610-T4PD and X626-T4P aluminum substrates immersed in 3.5% NaCl solution at 23C and 40C were measured. The measured results are shown inFig. 11. As shown inFig. 11(a), while pitting corrosion occurred in the polarization process of

aluminum substrate X610-T4PD at 23 C, little pitting

corrosion (i.e., a smooth curve) was observed for the anodic curve at 40 C. Furthermore, it can be seen from the polari-zation curves that the elevated temperature (i.e., 40 C) ac-celerates the corrosion of aluminum. For X626-T4P aluminum

shown in Fig. 11(b), although the temperature slightly

decreased the self-corrosion current, the corrosion current rapidly increased after the passivation film of X626-T4P aluminum was generated. These results suggest that the tem-perature accelerates the corrosion of X626-T4P aluminum.

Therefore, it can be deduced that the temperature likely in-creases the corrosion resistance of aluminum substrates, and consequently speeds up the degradation in joint strength. To validate this, the joints were exposed to 98% R.H. at 23C and 40C for 240 h and then tested. The results are presented in

Fig. 12. As shown, the decrease in strength of the

adhesive-bonded joint exposed to 98% R.H. at 23 C was smaller

than that exposed to 98% R.H. at 40 C. Furthermore, the

areas of adhesive failure of the adhesive-bonded joint exposed to 98% R.H. at 23C and 40C were examined and shown in

Fig. 13. It can be seen fromFig. 13that the degree of adhesive failure (i.e., 79.1%) of the joints exposed to 98% R.H. at 40C was significantly greater than that exposed to 98% R.H. at 23 C (i.e., 55.3%), which reveals that the temperature of

40 C under the hot-humid condition (i.e., 98% R.H. and

40C) leads to a more serious degradation in failure mode. These results suggest that the elevated temperature (i.e., 40C) speeds up the degradation in strength and failure modes of the adhesive-bonded aluminum joints.

Fig. 10. Oxygen content of 1.0 mm thick Novelis X610-T4PD (a) As-received (b) On the exposed fracture surfaces.

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4. Conclusions

Experiments were conducted to characterize the effect of hot-humid exposure on the quasi-static strength of adhesive-bonded lap-shear 1.0 mm thick X610-T4PD and 0.9 mm thick X626-T4P aluminum joints. The following conclusions are achieved:

1) Hot-humid exposure (i.e., 40C and 98% R.H. for 240 h) significantly decreased the joint strength and changed the failure mode from a mixed cohesive and adhesive with cohesive failure being dominant to adhesive failure being dominant.

2) The degradation in joint strength and failure mode were primarily attributed to the polar component of work of adhesion between adhesive and aluminum being destroyed by moisture and corrosion of aluminum substrate in the overlap region of the bonded joints, which degraded both polar and dispersion components of work of adhesion between adhesive and aluminum substrate.

3) The elevated temperature significantly accelerated the corrosion reaction of aluminum, which speeds up the degradation in joint strength.

Acknowledgments

This work was funded by General Motors Global Research and Development Center (Grant No.: PS21025708). The au-thors gratefully acknowledge Robert Szymanski for material preparation and handling.

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